It's probably because they're making them in much more limited quantities and don't expect them to sell extremely well, so the $80 is to cover inventory risk. Which is sorta counterintuitive since more people would buy it if it didn't require that $80.

That is one of the basic ideas behind supply and demand. You will often see different clothing colors priced differently. Same materials, same exact cost, but people are willing to pay more for black than white.

The guy in the Regents Street Apple Store said the material was just different. He said that it was slightly heavier than the white one. He also pointed out that Apple have been making white machines for five years and that, if there were any manufacturing snafus, they'd likely be with the black model.

Looked more matte in "person" than the pictures. While they had a number of demo machines, they'd sold out of them anyway. They seemed to have lots of the white ones left however, one of which my friend took. (Much as I love my 1st Gen iMac G5, I'm so jealous!)